Drilling rigs for drilling boreholes into the ground come in all sorts of different sizes. There are very large drilling rigs used to drill below twenty thousand feet which cost thousands of dollars per day to operate. There are very small drilling rigs designed to drill four or five thousand feet into the earth which are relatively inexpensive to operate. There are many other different sizes of drilling rigs designed for drilling to depths between five thousand and twenty thousand feet. A borehole drilled with a small drilling rig costs very little per foot of depth as compared to the same borehole drilled with a large drilling rig.
In drilling a deep borehole, the first few thousand feet of the wellbore is made very large in diameter as compared to the last few thousand feet of the wellbore. It really does not matter whether a large expensive drilling rig or a small relatively inexpensive drilling rig makes the first few thousand feet of hole. However, in order for a small drilling rig to bore a large diameter wellbore, it is necessary to utilize a large drill bit. In order to pass the large drill bit down through the turntable and the rotating blowout preventor, hereinafter called a RBOP, the RBOP and turntable must have an inside diameter which will accommodate the large drill bit. Otherwise, the above surface drilling equipment must be sized much larger than would ordinarily be used in conjunction with a small drilling rig.
It would be desirable to have an adaptor for a RBOP which could accommodate a drill bit of any size so that a relatively small drilling rig could drill a very large borehole. An improved RBOP which attains this desirable goal is the subject of the present invention.